1. Field of the Invention
This invention relates to the improvements in apparatus for processing of wafers of semiconductor material by thermal gradient zone melting and, in particular, to a guard ring for supporting and minimizing distortions in the thermal gradient in the wafer.
2. Description of the Prior Art
In the manufacture of semiconductor devices, it is normally necessary to alter the conductivity type of one or more selected regions of a semiconductor body by doping these regions with conductivity-modifying impurity atoms. Today, such doping is usually accomplished commercially by solid state diffusion, ion implantation, liquid epitaxial growth, or vapor epitaxial growth. Such factors, as costs, speed, junction characteristics, and the particular type of semiconductor material being used, determine which method is most practical.
A little used and less widely known technique for doping semiconductor material is thermal gradient zone melting. This technique can produce very abrupt junctions with unusual configurations and high dopant concentrations in a body of semiconductor material in a relatively short period of time. Early descriptions of such thermal migration and some of its applications are found in U.S. Pat. No. 2,813,048, issued to W. G. Pfann and in his book "Zone Melting", copyright by John Wiley and Sons, Inc. While the basic principle of thermal migration was known very early in the life of the semiconductor industry, a number of unsolved problems prevented its use as a standard processing technique by the semiconductor industry.
Thermal gradient zone melting (TGZM) is a process in which a small amount of dopant is disposed on a selected surface of a body of semiconductor material and the processed body is then exposed to a temperature gradient at an elevated temperature. The overall temperature at which the process is carried out must be sufficiently high in order to form a melt of metal-rich semiconductor material containing the dopant material. Under these conditions, the melt will migrate through a solid body of material along and up the lines of heat flow (a thermal gradient) from low temperatures to high temperatures, leaving in its path a recrystallized region of semiconductor material containing the solid solubility limit of the metal therein which includes the dopant material. The temperature gradient must be uniform and unidirectional if the pattern of dopant material is disposed on the surface area which is on the entrance face of the wafer is to be faithfully reproduced as a recrystallized dopant zone or region in the semiconductor wafer.
One of the most difficult problems which appears to be preventing the widespread use of thermal gradient zone melting has been the inability for one to be able to generate a large uniform thermal gradient across the thickness of a thin fragile semiconductor wafer without fracturing the wafer or contaminating the wafer with undesirable impurities.
A number of means of applying a large uniform thermal gradient have been tried including a plasma torch, a gas torch, a solar mirror, a scanning electron beam, a heated anvil and infrared radiation. The most satisfactory method of those tried has been to expose one side of a semiconductor wafer to a widespread intense source of infrared radiation while at the same time exposing the opposing side of the wafer to a cold black body heat sink. For a complete description of the infrared radiation method, attention is directed to the teachings of John K. Boah, entitled "Temperature Gradient Zone Melting Utilizing Infrared Radiation", U.S. Pat. No. 4,001,047, and assigned to the same assignee as this application.
Although the infrared radiation method of Boah produces a uniform thermal gradient through most of a semiconductor wafer, it has been discovered that around the peripheral edges of a wafer the thermal gradients are severely distorted from their otherwise unidirectional direction, which is perpendicular to the two major opposed surfaces, in the rest of the wafer by the discontinuity associated with the peripheral edge of a wafer. On first examination, it would appear that this thermal gradient distortion should only extend inwardly the equivalent of several wafer thickness from the edge of a wafer.
With reference to FIG. 1, there is shown a wafer 110 of semiconductor material processed in the prior art by thermal gradient zone melting. The wafer 110 has opposed major surfaces 112 and 114. Migration of one or more melts of metal-rich semiconductor material is from surface 112 to surface 114 when the surface 114 is exposed to infrared radiation. The infrared radiation of Boah produces radiation 116 which is incident upon the surface 114 and travels through the wafer 110 and is reradiated from the surface 112 and edges 128 of the wafer 110 as flow lines 122. The loss of heat from the edge or edges 128 of the wafer 110 distorts the heat flow lines 120 from a course directly between and perpendicular to the major surfaces 112 and 114 to an angled course of travel. That is the heat flow lines 120 deviate from the normal to the surfaces 112 and 114, and are not parallel with each other. Such non-parallel heat flow 120 will distort, and in some instances, break up any liquid alloy melt zone migrating through the material regions of distorted heat flow in the wafer 110. Only the area, or volume of material, in the center of the wafer where the heat flow lines 120 are substantially parallel to each other and perpendicular to the major surfaces 112 and 114 of the wafer 110 is useful for commercial semiconductor processing. However, we have found experimentally that for a wafer 110 with a radius of 25.4 mm, and a thickness of 0.25 mm, that the distortion of the thermal gradient generated by a heat loss around the edge 28 of the wafer 110 extends inwardly toward the center a distance of about 3 mm from the edge 128 of the wafer 110. Thus, the area over which the thermal distortion occurs represents about twenty percent of the area and volume of the wafer 110. Semiconductor devices made within this area, or volume of material must be discarded in most cases, thereby reducing processing yields and increasing processing costs. Consequently, a strong commercial incentive exists to find a practical means of eliminating the thermal distortions in the area contiguous with the peripheral edge 28 of the semiconductor wafer 110.
In U.S. Pat. No. 4,035,199, we have previously disclosed a method by which such thermal distortion can be minimized around the edge of a semiconductor wafer by having an annular guard ring configuration of a layer of absorption enhancing material disposed on the outer peripheral portion of the semiconductor wafer facing the source of the infrared radiation, and by having a centrally oriented disc-like configuration of a layer of emission enhancing material on the opposing face of the wafer facing the heat sink. This configuration is engineered so that additional heat is delivered to the otherwise relatively outer cold peripheral areas of the wafer while additional heat is drained from the otherwise relatively hot central portion of the wafer. The additional injection and extraction heat from these critical wafer areas minimized the heat flow tendencies shown in FIG. 1 and thus enabled one to use a larger percentage of the wafer area for device production. This result, however, was not obtained without incurring any disadvantages for a number of process steps had to be added to the overall process both to form and to configure the emission handling and absorption enhancing layers, thereby directly increasing production costs. In addition, for certain devices, the formation and configuration of these optical layers is not feasible since the various thermal anneals, chemical etches and masking steps involved either degrading or destroying the devices that one is attempting to produce.
In U.S. Pat. No. 3,895,967, we have previously disclosed a method by which thermal gradient distortions can be minimized around the edge of a thick semiconductor ingot as opposed to a thin semiconductor wafer. This method employed a guard ring of semiconductor material of the same thickness as the semiconductor ingot disposed about, and spaced from, the peripheral edge of the semiconductor ingot. This guard ring ingot arrangement adjusted the thermal distortion problem radially outward into the guard ring which could be re-used over and over again and eliminated thermal gradient distortions in the semiconductor ingot that was being processed. One requirement of this method was that the space or gap between the guard ring and the semiconductor ingot has to be less than one-tenth of the thickness of the semiconductor ingot. Otherwise, the guard ring becomes less effective and thermal distortion problems still are present in the peripheral edge portion of the semiconductor ingot.
For thin semiconductor wafers, the requirement that the separation width between the guard ring and the wafer be less than the wafer thickness and the requirement that the guard ring and the semiconductor wafer be co-planar make use of guard rings commercially unfeasible for a number of reasons. First, the wafer must be positioned in the guard ring without touching the guard ring. For small separations, this becomes exceedingly difficult and time consuming for mass production operations. Furthermore, the diameter of the wafers tend to vary from one lot to another requiring the costly manufacture of semiconductor guard rings for each new wafer lot. In addition, for thin wafers it is also difficult to align reproducibly the planes of the guard ring and the wafer. Without such parallel alignment, the guard ring method will not work effectively or may even be a complete failure.
In summary, then, the present methods of thermal gradient zone melting processing of thin semiconductor wafers either makes impossible the production of certain types of devices or wastes about twenty percent of a processed semiconductor wafer which must be discarded because of thermal gradient distortion problems around the peripheral edge portion of the wafer. In particular, the potentially attractive and simple prior art guard ring method in thick semiconductor ingots has been found to be commercially unfeasible for use in processing thin wafers of semiconductor material.
An object of this invention is to minimize the distortion of the thermal gradient and heat flow lines in a wafer of semiconductor material during thermal gradient zone melting processing.
Another object of this invention is to provide a guard ring for supporting and minimizing the distortion of heat flow lines in the outer peripheral edges of a wafer of semeconductor material.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.